If you find another user a) you can try charging and b) you can test it’s capacity
On the charging regime. The killer is heat so things to avoid are
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Allowing the battery to get low as it has to work harder to provide power at a lower voltage i.e. more amps.
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The battery is rated at 3.85V and from my own tests below that is stressful and that’s around 50% (2.5 year old battery)
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Rapid charging clearly will create more heat
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Do not charge overnight as you cannot stop it charging to 100%
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If you do want to charge overnight use a slow charger
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Again, from my tests, the maximum optimum range is 40% to 90%
You can try and clean the bottom module,